My ‘92 C1500 5.7/NV3500 has a 3-wire O2 in the Y pipe. My ‘94 K2500 has a 1-wire O2 in the Y pipe, same as my ‘95 C2500.Ya know, I've had the exact opposite observation. My '94 C2500LD 5.7/4L60E had a 3-wire O2 just ahead of the cat. My '94 C1500 5.7/NV4500 had a 1-wire O2 in the driver side manifold. Did you mistakenly get those examples backwards? The last few years of manual trans TBI stuff always seemed to me to have the 1-wire O2.
Richard